Retroreflective articles have the ability to return incident light back in the direction from which the light originated. This unique ability has led to the widespread use of retroreflective articles on a traffic signs. At nighttime, light from motor vehicle headlights is retroreflected by the articles to display information on the sign to motor vehicle drivers. During daylight hours, if the retroreflective articles have a sufficient daytime lightness to make the sign readily noticeable to passing motorists, daytime motor vehicle drivers may become privy to the same information. An ongoing goal in the retroreflective art, therefore, has been to provide retroreflective articles that demonstrate good nighttime retroreflectivity and sufficient daytime lightness to make the articles readily noticeable to passing motorists at any time of the day.
An example of a commonly used and good performing retroreflective article is a cube-corner retroreflective sheeting. A cube-corner retroreflective sheeting employs a multitude of cube-corner or prismatic elements to retroreflect the incoming light. Each cube-corner element has three planar faces that meet at an apex. At the base of each element is a sheet-like body portion (also known as a "land") which unites the cube-corner elements in the form of a sheeting. Light which is retroreflected typically enters the sheeting through a cover layer, passes through the body portion to strike the cube surfaces, and is reflected back in the direction from which the light entered the sheeting. A color is often provided to a cube-corner sheeting by incorporating a dye or transparent pigment into the body portion or cube-corner elements.
A fairly common cube-corner retroreflective sheeting has a metallic coating applied to the backside of the cube-corner elements to promote retroreflection; see for example, U.S. Pat. No. 3,684,348. While it is known that the metallic coating can improve the sheeting's retroreflective performance, it is also known that the metallic coating can have a detrimental effect on daytime lightness. The metallic coating, which is often aluminum, imparts an undesirable grayish daytime color to the sheeting, making information on the sign more difficult to read. This drawback makes metallic coated cube-corner sheetings much less likely to be used on signs that require good daytime lightness.
To improve the daytime lightness of metallic coated, cube-corner sheetings, the inventors of U.S. Pat. No. 4,801,193 developed a method of removing a predominant portion of the metallic coating from the back side of the cube-corner elements. This removal is accomplished by superimposing a second coating material over a minor portion of the metallic coating in a grid-like pattern and then exposing the coated cube-corner elements to a solvent to remove the metallic coating from the non-superimposed areas. A metallic coating in the form of a grid results. A backing element is then adhered to the backside of the cube-corner sheeting along the grid lines but is spaced apart from the cube-corner elements over the areas where the metallic coating has been removed. The patent discloses that upon observation of the sheeting in ambient light, a grayish grid pattern may be seen, which represents the residual metal on the backside of the cube-corner elements. Lighter areas between the grid lines are displayed which are characteristic of the areas where the metallic coating has been removed.
Another known approach for improving daytime lightness in a cube-corner sheeting involves placing opaque pigment particles that scatter white light in a base layer of the cube-corner sheeting. This approach does not rely on a metallic coating to promote retroreflection but instead employs a plurality of hermetically-sealed air chambers, which allow air to interface with the back side of the cube-corner elements without interference from moisture, dirt, and the like. A retroreflective article having this construction is shown in FIG. 1 where numeral 10 designates the retroreflective article.
Retroreflective article 10 includes a cube-corner sheeting 12 that has cube-corner elements 14 and a body portion 16. The body portion 16 has a smooth surface 18 through which light (such as from an automobile headlamp) passes in the direction of arrows 20 and 21. The incident light 20 strikes surfaces of cube-corner elements 14 and is reflected back in the direction 21 from which it originated. Air in hermetically-sealed air chambers 22 interfaces with the back side 27 of cube-corner elements 14 to promote the retroreflection of the incident light. Sealed chambers 22 are enclosed by a base layer 26 that is secured to sheeting 12 at bonding regions 24.
Retroreflective article 10 exhibits good daytime lightness by having opaque, white pigment particles 28 dispersed in the base layer 26. In the base layer's bonding region 24, the pigment particles 28 scatter the incident light to display a relatively light daytime color to the viewer. The cube-corner element/air interface also assists in improving lightness of the article 10. An article having this kind of construction has been sold for many years by 3M as Scothlite.TM. Diamond Grade Reflective Sheeting.